1. Field of the Invention
Subject matter of the invention is a method for preparing primary long-chain alcohols having on average greater than 20 carbon atoms.
2. Description of the Prior Art
Behenyl alcohol is a linear long-chain fatty alcohol of the formula C22H45OH, however, fatty alcohol mixtures having a chain length range of C18 (octadecanol) to C22 (docosanol) are also referred to as behenyl alcohols. On an industrial scale, these are prepared by means of fatty acid or fatty acid ester high-pressure hydrogenation. Raw materials for this are in particular erucic, crambe, or fish fatty acid, or their esters, which are obtained by hydrolysis or transesterification of the respective fats. One disadvantage of this method lies in the fact that raw materials from natural sources always provide fatty acid mixtures, consequently, as a matter of principle, the preparation of a certain individual component is coupled to the presence of additional components. Furthermore, this method is limited to chain lengths up to C22, with a few exceptions, due to the natural availability of the fatty acids.
According to another method, fatty alcohols are obtainable in the chain length range of C20 to C34 in accordance with US 2002/0099099-A1. Here, the fatty alcohol mixtures are obtained from natural products, preferably beeswax, by extraction and purification in organic solvents. Alcohol yields of at most 10 to 15% may be obtained if the esters contained in the beeswax are saponified prior to extraction.
Long-chain fatty alcohols up to a chain length of approximately C30 are also produced on an industrial scale by means of Ziegler's method (H. Ridder, K. Noweck, Ullmann's Encyclopedia of Industrial Chemistry, Fatty Alcohols, Fifth Edition, Vol. A10, 277-296 (1987)) starting from aluminum, hydrogen, and ethylene. Here, triethylaluminum is subjected with ethylene to a growth reaction, by which is meant the stepwise insertion of ethylene into the aluminum alkyl group; and after oxidation to the aluminum alkoxide and hydrolysis, C2 to C30 fatty alcohol mixtures are obtained, the fatty alcohols being present in Poissons's distribution. In methods carried out on an industrial scale, the distribution curve has a maximum at C10 to C12; however, it can be shifted to lower or higher mean molecular weights through the amount of ethylene used. The raw alcohols are subsequently distilled and separated into mixtures or individual fractions up to a chain length of C18 and C20, respectively. The portion accumulating in the bottom during distillation contains an alcohol distribution with a maximum at C20 and an alcohol content of approximately 80 wt %, or a maximum at C22 with an alcohol content of approximately 65 wt %. The raw alcohol obtained with Ziegler's method contains impurities, such as for example paraffins, olefins, ethers, esters, and aldehydes.
According to U.S. Pat. No. 3,255,256, alcohol mixtures, such as for example obtained in Ziegler's process after oxidation and hydrolysis, are converted to aluminum alkoxides in order to then separate the more volatile impurities by distillation or stripping. In this manner, paraffins, olefins, ethers, esters, and aldehydes are successfully separated from alcohols up to a chain length, above which, due to the high boiling points of the secondary components, a thermal decomposition of the alkoxide takes place. The subsequent hydrolysis of the aluminum alcoholates with aqueous systems yields a raw alcohol mixture and aluminum hydroxide.
This method is applicable for the separation of the non-alcoholic secondary components of Ziegler's method; however, it is not suitable for the separation of linear alcohols from branched alcohols. Ziegler raw alcohol contains with growing chains length also an increasing content of predominantly 2-branched alcohols. Analysis of the alcohol fraction of a typical Ziegler alcohol (NAFOL® 20+) after separation using column chromatography gives for example at a chain length of C20 7.8%, at C22 8.0%, and at C24 already 17% of branched alcohols (Table 1).
TABLE 1Example of an Alcohol Distribution of the AlcoholFraction in NAFOL ® 20+ChainName ofn-Alcoholiso-AlcoholRelative ProportionLengthn-AlcoholMass-[%]Mass-[%]iso-Alcohol [%]C18Octadecanol20 < 0.1%—C20Eicosanol40.43.47.8C22Docosanol28.72.58.0C24Tetracosanol8.81.817.0C26Hexacosanol4.31.221.8C28Octacosanol2.10.930.0C30Triacontanol1.00.637.5C32Dotriacontanol0.50.337.5Total87.810.710.9
During the growth reaction in Ziegler's process, ethylene is stepwise inserted into the aluminum carbon bond of the aluminum alkyls. Starting with triethylaluminum, longer chain aluminum alkyls with an even number of carbons are formed. One of the side reactions is the thermal cleavage of α-olefins, preferably ≧C4, with simultaneous formation of a dialkylaluminum hydride. The olefin cleavage is an equilibrium reaction, i.e., the dialkylaluminum hydride, in turn, can react with ethylene or the α-olefins with chain lengths of ≧C4 to form a trialkylaluminum. Just as in the growth reaction with ethylene, the formed α-olefins can also insert into the aluminum carbon bond. This hydroaluminization reaction takes place regioselectively, preferably with formation of a 2-alkyl-branched ligand which in turn is cleaved off forming a branched olefin. Branched ligands that are bound to the aluminum may also continue to grow with ethylene to longer chains, the branching site then removing itself by two carbon atoms at a time from the aluminum atom.